1. Field of the Invention
The present invention relates to an electrode structure for a p-type Al.sub.x Ga.sub.y In.sub.1-x-y N (0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1, x+y.ltoreq.1) semiconductor device, one of the Group III-V compound semiconductor devices containing nitrides, and a method for fabricating the same. More specifically, the present invention relates to an electrode structure having an ideal ohmic contact showing an extremely small contact resistance between a semiconductor layer and an electrode layer and a method for fabricating the same.
2. Description of the Related Art
Generally, in fabricating an electrode structure for an Al.sub.x Ga.sub.y In.sub.1-x-y N (0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1, x+y.ltoreq.1) semiconductor device, nitrogen, one of the elements constituting the semiconductor, is likely to dissociate from the surface of a semiconductor layer in the electrode structure when the semiconductor layer is formed. Therefore, it is difficult to produce crystals satisfying a desirable stoichiometric ratio. When the dissociation of nitrogen forms the vacancies inside the crystal structure of the semiconductor layer, the conductivity type of the semiconductor layer turns into n-type. Therefore, in fabricating an electrode structure for an Al.sub.x Ga.sub.y In.sub.1-x-y N (0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1, x+y.ltoreq.1) semiconductor device, it is difficult to form a p-type semiconductor layer.
As a method for turning a semiconductor layer containing nitrogen into a p-type semiconductor layer, it is well known to dope the semiconductor layer with magnesium (Mg) as an acceptor impurity. However, a p-type Al.sub.x Ga.sub.y In.sub.1-x-y N (0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1, x+y.ltoreq.1) semiconductor layer formed by a commonly-used metalorganic chemical vapor deposition (MOCVD) method contains a large amount of hydrogen inside semiconductor crystals. A part of the hydrogen atoms are bonded with the Mg atoms functioning as an acceptor impurity, thereby preventing the Mg atoms from functioning as an effective acceptor. In order to activate the Mg atoms as an acceptor impurity, the semiconductor layer is subjected to an electron beam irradiation process or an annealing process within a nitrogen environment, thereby forming the p-type Al.sub.x Ga.sub.y In.sub.1-x-y N (0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1, x+y.ltoreq.1) semiconductor layer. However, the p-type semiconductor layer subjected to such a process does not have a carrier density high enough to form an ideal ohmic contact between a semiconductor layer and an electrode layer.
On the other hand, various electrode structures usable for Group III-V compound semiconductor devices containing nitrides such as blue-light-emitting diodes have been conventionally developed. In the proposed electrode structures, various kinds of metals are used to form the electrode layer. For example, in order to form the electrode layer for a p-type Al.sub.x Ga.sub.y In.sub.1-x-y N (0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1, x+y.ltoreq.1) semiconductor device, gold (Au) is most commonly employed ("P-GaN/N-InGaN/N-GaN Double-Heterostructure Blue-Light-Emitting Diodes", S. Nakamura et al., Jpn. J. Appl. Phys. (1993) p.L8). Japanese Laid-Open Patent Publication No. 5-291621 discloses that nickel (Ni), platinum (Pt) and silver (Ag) may be used in place of Au as the metals for forming the electrode layer.
However, in the case of using Au for the electrode layer, the contact resistance between the electrode layer and the semiconductor layer is large, and therefore an ideal ohmic contact cannot be obtained. In addition, the adhesiveness between the electrode layer and the semiconductor layer is inferior and the physical strength of the semiconductor device becomes disadvantageously weak.
On the other hand, in the case of using Ni, Pt or Ag for the electrode layer, the resulting adhesiveness is surely superior to that of Au, so that a more ideal ohmic contact can be obtained as compared with the case of using Au. However, in the case of using these metals, the following problems are caused, for example. In a light-emitting diode using these metals for the electrode layer, a differential resistance value at the current value of 10 mA is large, i.e., several tens of .OMEGA.s. In other words, such a light-emitting diode has a high operational voltage, judging from the current-voltage characteristics thereof. In addition, since a laser diode using these metals for the electrode layer has a small electrode area, the contact resistance is increased as compared with a light-emitting diode. As a result, the operational voltage of the laser diode becomes larger than that of a light-emitting diode. This is why an electrode structure having a sufficiently ideal ohmic contact cannot be obtained even by the use of these metals.
In consideration of these problems, a conventional electrode structure using these metals for the electrode layer cannot be regarded as an ideal electrode structure for a p-type semiconductor device. Therefore, an electrode structure for a p-type semiconductor device having an ideal ohmic contact showing an extremely small contact resistance between a semiconductor layer and an electrode layer is eagerly sought.